Method and apparatus for cascaded biomass oxidation with thermal feedback

ABSTRACT

The invention relates to a method for cascaded biomass oxidation in a dish burner with ejection firing. The fuel ( 18 ), together with oxygen-containing primary air, is fed to a gasifier dish ( 8 ) having high thermal conductivity, in which the fuel is gasified by pyrolysis in a first combustion step, the resulting gas is conducted through guiding devices ( 6, 9 ) over the dish edge ( 11 ) of the gasifier dish, or over recesses on the upper dish edge, to the outside wall of the dish ( 8 ), and is enriched with oxygen-containing secondary air in the intermediate chamber ( 10 ) and converted into a cyclone flow around the outer shell dish during a second combustion step, through the convection of which strong thermal feedback is created, together with the high reflection of the thermal radiation on the guiding devices.

The present invention concerns a method for cascaded biomass oxidationin a dish burner with ejection firing and an apparatus for carrying outthe method. It concerns the field of heating technology, in particularfurnace technology, for solid fuels, with a focus on biomass combustionsystems, preferably those for pellets and wood chips.

This method and the apparatus are preferably used for water heating, forheating, and to a small extent for supplying power through cogeneration,or combined heat and power, in single-family and multi-family homes. Themethod serves the purpose of optimum combustion of fuels in a dishburner with ejection firing, preferably in burner systems for pelletsand wood chips. The apparatus represents the central device of a heatingboiler, and permits compact, efficient, and low-residue combustion inorder to obtain heat.

PRIOR ART

Modern burner systems are divided primarily into top-fed systems withand without grates, stoker feed systems, and retort burners.

In the first type, the fuel (pellets, wood chips, shavings, grain andthe like), after being delivered through a feed auger, falls through achute onto a grate or a firebox with the fire bed. Initial ignition isaccomplished by a hot air blower or electric heating devices. The flamesreach upward and the hot flue gases are discharged upward to the heatexchanger through flues. These systems are not suitable for straw-likematerials. Fill level monitoring is difficult (visually or by lambdaprobe). A horizontal arrangement of the combustion chamber is referredto as a tunnel burner, which likewise functions without a grate. Thepellets are gasified in the combustion zone with the aid of primary air.The secondary air is introduced into an attached combustion cylinder orthrough laterally arranged nozzle bores. As a general rule, a relativelysmall intake air flow is additionally introduced through the chute inorder to reduce the risk of burn-back. In tilting and vibrating gratesystems, the quantity of ash that is produced is automatically droppedperiodically into the ash collector located underneath.

In general, however, a pellet stove or furnace assumes a leadingposition with regard to low emissions and high efficiency, not least onaccount of the high homogeneity of the fuel; the carbon monoxideemissions are far lower than for other individual heat-producingappliances, and the efficiency reaches values of over 90%.

Stoker feed systems are subdivided into lateral feed (transverse feed)and underfeed systems, where stoking takes place from the side or frombelow to a grate (fixed, possibly with a tilt function, or traveling asa moving grate) or a steel plate (as a moving floor with or withoutwater-cooling). In the case of transverse feed burners, a portion of thecombustion air is blown in through the grate when present, through airnozzles in the side region of the burner trough, or—in the case ofmoving grate burners—through end-face air channels on the grateelements. The primary air also fulfils the function of grate coolinghere. The secondary air is delivered above the grate or fire bed orahead of the entrance to the secondary combustion chamber.

In retort burners, the fuel is supplied from below the retort (fueltrough) by means of a stoking auger. The drying, pyrolyticdecomposition, and gasification of the fuel take place in the retort, inaddition to burn-off of the charcoal by means of primary air that isblown in. The secondary air is mixed with the combustible gases ahead ofthe entrance to the hot secondary combustion zone. Advantages here arethe low inertia and the residual heat, although disadvantages includehigh wear of steel parts, inhomogeneous fire bed and fuel compaction,high residual fuel content in the ash, and high pollutant productionduring the shutdown process.

For the combustion principles, a distinction is drawn betweenburn-through, top firing, and bottom firing. In burn-through, combustionair is passed through a grate and the fuel layer. A disadvantage here isthe difficult adaptation of the quantity of combustion air to thedifferences in combustion gas release.

In top firing, the combustion air arrives laterally at the fire bedzone.

In bottom firing, only the bottommost layer of the fuel takes part inthe combustion. The combustion gases released in the region of theprimary air feed are diverted by a forced draft to the combustionchamber where they undergo secondary combustion under the supply ofsecondary air. If the combustion chamber is at the bottom, it is calleddowndraft combustion, if the combustion chamber is to the side, it isreferred to as lateral bottom combustion. In general, the controlvariable for regulating the different combustion phases (start-up phase,stationary phase with constant output, partial load phase and burnoutphase) is the air. When there is a separation between primary air flowand secondary air flow, there are two control variables. The burneroutput can be adjusted from approximately 50% to 100% with the primaryair. The complete burn-off of the combustible gases is controlled withthe secondary air. For output-regulated boilers, an induced-draft orforced-draft blower serves to control output. The fuel feed is regulatedat the same time. Lambda probes are used to measure the excess air inthe exhaust gas flow.

DE 10 2005 033 320 dated Feb. 15, 2007 discloses a method for burningsolid fuels and a device according to that method. Shown here is atray-like grate with at least one air chamber, wherein primary air isfed into this air chamber and secondary air is conducted around the airchamber. WO 99/15833 dated Apr. 1, 1999 shows a “Self-Stoking WoodPellet Stove” with the features: worm conveyor, discharge chute, andregulator valves for supply of primary and secondary air, as well as aflue leading upward.

Because of the asymmetrical arrangement of the elements, the abovesystems are beset by the following disadvantages: the difficulty ofachieving a state of optimal combustion even under partial load, thecomplex design or cleaning of the grate and the great space requirementfor the combustion device.

Each of the cited systems has advantages and disadvantages, with no oneof these systems combining the majority of advantages.

OBJECT OF THE INVENTION

Based on this prior art, a method and an apparatus were sought foroptimal combustion of biomass in which the advantages of all existingsystems are combined to the greatest possible degree and, moreover,which provide good results even in partial load conditions. The designshould be compact, the material requirements low, and the constructionshould be minimal with respect to the achievable performance yield. Inaddition, the number of wearing components should be reduced to anacceptable minimum, or else the service life of the wearing parts shouldbe as long as possible. Moreover, a very low-noise combustion system isdesired with the most homogeneous burn-off possible and low turbulencein the heating. Uncontrolled convection of the heat should be avoided tothe greatest extent possible. In like manner, sooting and burn-backshould be precluded. In addition, for maintenance, the easiest possibleaccessibility to the areas to be cleaned should be provided and simplemeans for replacement of parts should be provided.

Application for Protection

In order to attain this object, provision is made for fuel to besupplied together with primary air containing oxygen to a gasifier boxwith high thermal conductivity, wherein the fuel is gasified throughpyrolysis in the first combustion step, the resultant gas is directed tothe outside wall of the box by guide devices above the box edge of thegasifier box or through recesses at the top edge of the box to theoutside wall of the box, and is enriched in the intermediate space withsecondary air containing oxygen and, during a second combustion step, istransformed into a cyclonic flow about the outside box wall, the highconvection of which, in conjunction with the high reflection of the heatradiation at the guide devices, results in strong thermal feedback.

Here, fuel is supplied together with oxygen-containing primary air to agasifier box in a dish burner with ejection firing with high thermalconductivity, wherein the fuel is gasified through pyrolysis in thefirst combustion step. The resultant gas is directed by special guidedevices above the box and around the box. For example, due to thesuction caused by a suction fan or the pressure caused by a forced-draftblower, the gases travel across the box edge of the gasifier box orthrough recesses at the top edge of the box directly to the outside wallof the box. In the narrow-walled intermediate space, enrichment withoxygen-containing secondary air takes place, while a cyclonic flow aboutthe box outside wall is simultaneously imposed by suitable devices.During this process, intensive mixing and uniform distribution of theexothermically reacting gases takes place in the second combustion step.Reflections of the heat radiation at the guide devices and highconvection of the gas flow serve the purpose of strong thermal feedbackof the materials to one another, and hence to very good burnout of theflue gases. The pyrolysis is improved or can take place in a smallerspace. The dense and efficient gas conduction brings about a flue gaswith an extremely low concentration of pollutants, even under partialload conditions. Particles of ash are forced against the burner wall bythe cyclonic flow, and fall out of the main gas flow. Soot particles andfine particulates burn up at the hot guide devices because of theircatalytic effect.

It is especially advantageous when the predominant flow direction of thecyclonic flow points downward and the gas flow is conveyed directly intothe diffuser chamber at the lowest point directly beneath the box. As aresult, the fuel gases are guided downward in a targeted and homogeneousmanner, opposite to the customary upward chimney direction.

As a refinement, the flow can be forced into additional cascades withthe aid of alternating dish-like and funnel-like guide devices. Then anupwardly directed flow follows the downward flow again, and vice versa.The cascades thus produced represent flow layers with respect to thediffuser chamber, which can deliver heat to the environment or heatexchanger in an optimized manner.

It is advantageous if the flow speed of the primary air in the gasifierbox and the flow speed of the secondary air in the zones outside thegasifier box are different or can be regulated separately from oneanother.

In an apparatus according to the invention, guide devices are providedabove and below a gasifier box to guide the gas flow after pyrolysisinto close contact with the outside of the gasifier box, wherein atleast one secondary air nozzle is provided between the gasifier box andthe guide devices whose axial flow outlet direction has a horizontalcomponent and runs at least approximately tangentially to the gasifierbox wall.

Essentially, this consists of a two-dimensional upper boundary with theexception of the feed pipe, and a two-dimensional boundary arrangedlaterally around the gasifier box. The guide devices serve to guide thegas flow after pyrolysis into close contact with the outside of thegasifier box and to reflect the heat of combustion in alternationdepending on the operating state. A cyclonic flow is imposed with theaid of at least one secondary air nozzle between the gasifier box andthe guide devices. To this end, the axial flow outlet direction of thenozzle is advantageously such that it has a horizontal component. Thenozzle axis here runs at least approximately tangentially to thegasifier box wall.

A simple embodiment of the top guide device is in the form of a coverconstructed in two layers with a hollow space. This hollow space shouldhave at least one air intake opening. The bottom boundary of the covercan be connected to at least one nozzle whose air intake opening isconnected to the hollow space of the cover. A fill opening with feedpipe for the fuel is left free here. This feed pipe is also nearlygas-tight toward the top, with the result that the flue gas can onlyescape downward.

Guidance takes place by means of the bottom guide device, which, like afunnel, is designed as a second box around the outer wall of thegasifier box with the exception of a bottom recess, with parallelspacing or with spacing which changes toward the bottom. In this design,the bottom guide device is attached to the top guide device.

The bottom guide device, like the gasifier box, can have pyramidal,spherical, conical, paraboloidal or hyperboloidal surface segments.

The gasifier box and its surrounding guide device can advantageously bearranged coaxially and concentrically about a burner axis. They may bemade to be rotationally symmetrical.

It is advantageous for at least one air outlet opening of a primary airnozzle to be positioned inside the box just above the lowest point ofthe box, near the burn-off position of the biomass, by which means theoptimal quantity of air can be supplied to the pyrolysis. The positionof at least one air outlet opening, specifically of at least onesecondary air nozzle, should usefully be provided on the outside of thebox.

At least one of the air supply nozzles can be designed to good advantageas an annular gap nozzle about a feed pipe. Alternatively or inaddition, multiple primary air nozzles or multiple secondary air nozzlesmay be arranged about the burner axis at uniform angular spacing.

It is useful for the flow speed through the air supply nozzles to beregulated and controlled by at least one suction blower as exhaust gasfan. In addition, it can be regulated by means of a rotary valve aroundthe air intake openings.

Bimetallic regulators can advantageously be provided in the air supplynozzles for temperature-dependent regulation of the flow speed.

The air supply nozzles can be made by deep drawing, but can also be madefrom welded-on metal tube sections or ceramic parts.

It is useful for the top guide device to be spherical in shape to avoidstress deformations or cracks, or else to have individual, separateconvexities as an expansion reserve.

The radial spacing between the gasifier box and the bottom guide devicecan be made mutually adjustable. This spacing is constant in the axialdirection, or tapers in the flow direction.

To avoid heat losses at the top, a thermal insulating layer, preferablymade of mineral wool or ceramic fiber, is provided above the top guidedevice.

The gasifier box can be provided with a spring-mounted impact cone orgrate-like components for removal of residual ash.

The invention is explained in detail below with reference to theattached drawings. They show:

FIG. 1 a vertical section through the invention.

FIG. 2 the horizontal section A-A′ from FIG. 1.

In FIG. 1, the method and the apparatus are illustrated using thevertical section. Shown is the burner 1 with its double-box constructionthat is clearly visible here. Above the central feed pipe 2, the fuel18, preferably pellets, wood chips, or wood shavings, is fed into thegasifier box 8 with the aid of an auger (not shown here). The fuel fallsthrough the feed pipe because of gravity at intervals in time because ofthe auger advance. In doing so, it passes through the fill opening 20 inthe burner cover. The feed pipe has a flow resistance towards the topthat is accomplished through suitable check valves, through seals, orthrough a slight primary air flow on account of an overpressure onaccount of a forced-draft blower or a suction blower in the exhaustduct, not shown here. The fuel falls during the course of combustion,and the burned fuel is replaced by fresh fuel. Since the optimumoperating condition is achieved at a constant box level, even atdifferent outputs, and the fuel from above tends to be cold, the levelfor precise fuel metering can be achieved through detection by a sensor23, even with imprecise conveyor systems. In the case of a cold start,the fuel 18 consisting of biomass is heated by electric heating elementsor by hot air. This initial heat can be supplied from above through theprimary air nozzles 3 with integrated glow wires or from below through apossible grate in order to start the pyrolysis process. In FIG. 1, fourprimary air nozzles 3 are shown (three visible), which are arrangedabout the burner axis 19 at an angle of 90° and extend diagonally intothe gasifier box 8. The biomass fuel 18 may also cover the primary airnozzles 3. The primary air can also be supplied through a possible gratemounted on the floor or in the vicinity of the floor of the gasifierbox. Because of the initial heat, the fuel is gasified by the pyrolysis,with additional heat being released in the process, with the result thatthe hot air or electric heat is no longer required. The gasificationproduces combustible gases consisting of carbon compounds, therebybringing about temperatures of up to 1000° C. at the floor of thegasifier box. The gases are directed by the blower up the inside wall ofthe box to the box edge 11, where transfer into the intermediate space10 occurs. If the box is suspended at multiple points or separated fromthe bottom guide device by spacers, then the flow slips directly overthe edge of the box. However, a direct connection of the box edge to thetop guide device 6, in particular its lower boundary 22, is alsopossible. In this case recesses, such as holes in the top edge of thebox, must be provided in order to allow flow into the intermediate space10 between the gasifier box and the bottom guide device 9. Here, thegasifier box represents the surface of a truncated cone that is made ofsheet steel approximately 2 mm thick, with a level floor. The angle ofopening is about 90° and the bottom guide device 9 forms a conicalfunnel at the radial spacing 12, and is displaced axially andconcentrically thereto. This produces a double wall with constantspacing in the axial direction. Between these walls, the primarycombustion of the pyrolysis gases takes place with secondary airsupplied through secondary air nozzles 13. The shape of the box does nothave to be conical, any box shape that forms a trough is possible, andonly minor restrictions are placed on the shape of the guide device 9 aswell. Here, the elements critical to the shape are the production ofthermal stresses, the desired flow profiles for the combustion gas flow,and the aspects of aging-induced wear and maintenance. Thus, spherical,conical, paraboloidal, hyperboloidal or even multi-sided pyramidalshapes or composite shapes are possible. When a spacing that narrowstowards the bottom is chosen, the rotational speed of the gas flowincreases towards the bottom. Also, a spiral-shaped guide groove may beprovided on the outside wall of the box and the inside wall of the guidedevice 9 in order to achieve dimensional stability coupled with laminarcyclonic flow. This cyclonic flow is also achieved by the shape of thesecondary air nozzles 13. Here, six nozzles are distributed evenly aboutthe box circumference in the region where the combustion gases from thegasifier box enter the intermediate flow space 10. The angular spacingis 60° here. The nozzle opening 15 in this design is not aimedperpendicularly downward, but instead is tilted at an angle to theburner axis 19. It is useful for this angle to be chosen such that thedwell time of the flow about the gasifier box brings about good feedbackor interaction with the first stage of combustion. The load isdetermined by the feed rate of the fuel 18 by the auger in conjunctionwith the strength of the blower draft and the removal of heat.Regardless of this, the gasifier box 8 should be kept at an optimumtemperature level as activation energy for the pyrolysis. The tightguidance of the secondary combustion air along the gasifier box 8,coupled with the high reflection of the heat radiation by the guidedevice 9 on the one hand and by the gasifier box 8 on the other hand,has a self-regulating effect, causing the gases to be fully oxidizedindependently of load before they can strike the cold diffuser surfacesand incompletely combusted pollutant gases such as carbon monoxide canexit the chimney.

The number and shape of the secondary air nozzles are chosen as desiredhere. The imagination is unfettered by restrictions. Thus, an annulargap nozzle can also open onto the intermediate space 10 formed by thebottom boundary 22 of the top guide device 6. Additional guide panelscan then bring about the defined cyclonic flow.

Here, the primary air and secondary air originate directly from thehollow space 6 of the two-layer top guide device (burner cover). The airis supplied from outside through the air intake opening (shown here as acircumferential gap). The conduction of heat by the panels employed andthe heat radiation of the combustion gases accomplish the result thatthe burner cover reaches high temperatures and thus also bring abouthigh preheating for the primary and secondary air. The primary airnozzles 3 and the secondary air nozzles 13 are connected directly to thehollow space 5 of the burner cover 6 through the air inlet openings 4and 14. Thermal losses are prevented by the insulating layer 17 abovethe burner cover 6.

FIG. 2 shows a view from above of the section along line A-A′ from FIG.1, and makes clear the symmetrical design of the embodiment. The airinlet openings of the nozzles 3, 13 are visible in cross-section, andthe gasifier box is visible.

LIST OF REFERENCE CHARACTERS

1 dish burner

2 feed pipe

3 primary air nozzle

4 air inlet opening in the primary air nozzle

5 hollow space of a two-layer cover

6 top guide device (cover)

7 air outlet opening from the primary air nozzle

8 gasifier box

9 bottom guide device (wall, cyclone nozzle, funnel)

10 intermediate space (2^(nd) combustion stage)

11 box edge of the gasifier box

12 spacing between box and bottom guide device

13 secondary air nozzle

14 air inlet opening in the secondary air nozzle

15 air outlet opening from the secondary air nozzle

16 air intake opening

17 insulating layer

18 fuel with ash from biomass

19 burner axis

20 fill opening for the fuel

21 opening of the bottom guide device (nozzle, funnel) into the diffuserchamber

22 bottom boundary of the top guide device (cover)

23 sensor for determining fuel quantity

1-20. (canceled)
 21. A method for cascaded biomass oxidation in a dishburner with ejection firing, the method comprising: supplying fueltogether with primary air containing oxygen to a gasifier box with highthermal conductivity; gasifying the fuel through pyrolysis in a firstcombustion step; directing the resultant gas towards the outside wall ofthe box via guide devices arranged above a box edge of the gasifier boxor through recesses arranged at the top edge of the gasifier box towardsthe outside wall of the box; and enriching the resultant gas in anintermediate space with secondary air containing oxygen and, during asecond combustion step, transforming the resultant gas into a cyclonicflow about the outside box wall, a high convection of which, inconjunction with a high reflection of heat radiation at the guidedevices, resulting in strong thermal feedback, wherein the flow can beforced into additional cascades with the aid of additional alternatingdish-like and funnel-like guide devices, and wherein a downward flow isfollowed again by an upward flow, and conversely an upward flow isfollowed again by a downward flow, and the cascades thus producedrepresent flow layers with respect to the subsequent diffuser chamber.22. The method according to claim 21, wherein a predominant flowdirection of the cyclonic flow points downward and the gas flow isconveyed directly into a diffuser chamber at the lowest point directlybeneath the box.
 23. The method according to claim 21, wherein the flowspeed of the primary air in the gasifier box and that of the secondaryair in the zones outside the gasifier box are different or can beregulated separately from one another.
 24. An apparatus for cascadedbiomass oxidation in a dish burner with ejection firing, the apparatuscomprising: guide devices arranged above and below a gasifier box toguide the gas flow after pyrolysis into close contact with an outside ofthe gasifier box; and at least one secondary air nozzle provided betweenthe gasifier box and the guide devices whose axial flow outlet directionhas a horizontal component and runs at least approximately tangentiallyto the gasifier box wall.
 25. The apparatus according to claim 24,wherein the top guide device is designed as a cover constructed in twolayers with a hollow space and has at least one air intake opening, hasat least one nozzle with air inlet opening that is connectable to the abottom boundary of the cover and to a hollow space, and has a fillopening with feed pipe for the fuel.
 26. The apparatus according toclaim 24, wherein the bottom guide device, with the exception of abottom recess, is designed as a second box around the outer wall of thegasifier box, with parallel spacing or with spacing that narrows towardsthe bottom, and wherein the bottom guide device is attached to the topguide device.
 27. The apparatus according to claim 26, wherein thebottom guide device and the gasifier box have pyramidal, spherical,conical, paraboloidal or hyperboloidal surface segments.
 28. Theapparatus according to claim 24, wherein the gasifier box and itssurrounding guide device are arranged coaxially and concentrically abouta burner axis and are configured to be rotationally symmetrical.
 29. Theapparatus according to claim 24, wherein at least one air outlet openinghas at least one primary air nozzle just above the lowest point of thebox inside the box, near the burn-off position of the biomass, and/orhas at least one air outlet opening of at least one secondary air nozzleon the outside of the box.
 30. The apparatus according to claim 24,wherein at least one of the air supply nozzles is configured as anannular gap nozzle about the feed pipe.
 31. The apparatus according toclaim 24, wherein multiple primary air nozzles and/or multiple secondaryair nozzles are arranged about the burner axis at a uniform angularspacing.
 32. The apparatus according to claim 24, wherein the flow speedthrough the air supply nozzles is regulatable by at least one suctionblower as exhaust gas fan and/or a rotary valve around at least one airintake opening.
 33. The apparatus according to claim 24, wherein theregulating devices are provided in the air supply nozzles.
 34. Theapparatus according to claim 24, wherein the air supply nozzles are madeby deep drawing, or from welded-on metal or ceramic tube sections. 35.The apparatus according to claims 24, wherein the top guide device isspherical in shape, or has individual, separate convexities.
 36. Theapparatus according to claim 24, wherein the gasifier box and the bottomguide device are arranged with a small, constant radial spacing, or aradial spacing that tapers in the flow direction, and wherein thespacing is adjustable via lifting spindles.
 37. The apparatus accordingto claim 24, wherein a thermal insulating layer made of mineral wool orceramic fiber is arranged above the top guide device.
 38. The apparatusaccording to claim 24, wherein the gasifier box has a grate or aspring-mounted impact cone for removal of residual ash.
 39. Theapparatus according to claim 24, wherein the mass flow rate of the fuelis regulatable by the level of the fuel at its input-side surface in thegasifier box via a sensor.